From DE 10 2013 114 138 A1, an analytical device is known for determination of a digestion parameter of a liquid sample, comprising at least one reactor and measuring device for determining the parameter of the liquid sample, a container device for storage of samples, reagents, and waste products in containers, a feed and metering device for metering and feeding the sample and reagents from the containers into a metering vessel and for disposal of waste products from the metering vessel into a waste container, and a sensing element for detecting a measured value, correlating with the parameter to be measured, of the liquid sample optionally mixed with one or more reagents in the reactor and measuring device. The feed and metering device consists at least of a metering vessel, a piston pump, and an additional sample collection device for at least taking a sample of a given volume of a liquid as a liquid sample from a sample collection site.
During the metering of the liquids, accurate determination of the volume of the liquids is of great significance. During metering, the respective liquid is drawn at high speed in the direction of the dose measuring device, until the metering unit activates. The liquid located in the metering unit is then forced at reduced speed below the dose measuring device. Then, the missing liquid is drawn in at low speed up to the dose measuring device.
Both during purging and during accurate metering for the measurement process, the dose measuring device may be activated not only by the liquid, but also by a liquid film surrounding an air bubble or by liquid films in the metering unit that form due to air bubbles. These air bubbles form whenever the liquid-carrying tubes are not completely filled with liquid or when the liquids or reaction products outgas. This is especially critical during the purging of the sample, since the sample tubing is empty before each measurement.
During the purging of the analytical device, these air bubbles can cause the metering device to not be completely rinsed and/or the measurement reactor to not be completely emptied. Because of the high feed velocity and the possibly high suction breadth or pressure, there is a risk that air bubbles will be drawn into the metering unit and erroneously activate the dose measuring device because of the film of liquid surrounding the air bubble or the liquid film formed in the metering device by the air bubbles.
In a measurement process, erroneous activation of the dose measuring device can lead to the volume to be metered not being set correctly. This will impair the measurement accuracy of the analytical device. Accordingly, there remains a need for further contributions in this area of technology for metering liquids.